1. Field of the Invention
This invention relates to electrically-pumped vertical-cavity surface-emitting lasers (VCSELs) and, in particular, to electrically-pumped VCSELs having current confinement.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Lasers have a wide range of industrial and scientific uses. There are several types of lasers, including gas lasers, solid-state lasers, liquid (dye) lasers, and free electron lasers. Semiconductor lasers are also in use. In semiconductor lasers, electromagnetic waves are amplified in a semiconductor superlattice structure. Semiconductor lasers may be diode lasers (bipolar) or non-diode lasers such as quantum cascade (QC) lasers (unipolar). Semiconductor lasers are used for a variety of applications and can be built with different structures and semiconductor materials, such as gallium arsenide (GaAs).
The use of semiconductor lasers for forming a source of optical energy is attractive for a number of reasons. Semiconductor lasers have a relatively small volume and consume a small amount of power as compared to conventional laser devices. Further, semiconductor lasers can be fabricated as monolithic devices, which do not require a combination of a resonant cavity with external mirrors and other structures to generate a coherent output laser beam.
A semiconductor laser typically comprises an active (optical gain) region sandwiched between two mirrors, one of which serves as the “exit” mirror. When the active region is pumped with an appropriate pumping energy, it produces photons, some of which resonate and build up to form coherent light in the resonant cavity formed by the two mirrors. A portion of the coherent light built up in the resonating cavity formed by the active region and top and bottom mirrors passes through one of the mirrors (the exit mirror) as the output laser beam.
Various forms of pumping energy may be utilized to cause the active region to begin to emit photons. For example, semiconductor lasers of various types may be electrically pumped (EP) (by a DC or alternating current), or pumped in other ways, such as by optical pumping (OP) or electron beam pumping. EP semiconductor lasers are typically powered by applying an electrical potential difference across the active region, which causes a current to flow therein. As a result of the potential applied, charge carriers (electrons and holes) are injected from opposite directions into an active region where recombination of electron and holes occurs. There are two kinds of recombination events, i.e. radiative and non-radiative, concurrently happening in the active region. When radiative recombination occurs, a photon is emitted with the same energy as the difference in energy between the hole and electron energy states. Some of those photons travel in a direction perpendicular to the reflectors of the laser. As a result of the ensuing reflections, the photons can travel through the active region multiple times.
Stimulated emission occurs when radiative recombination of an electron-hole pair is stimulated by interaction with a photon. In particular, stimulated emission occurs when a photon with an energy equal to the difference between an electron's energy and a lower energy interacts with the electron. In this case, the photon stimulates the electron to fall into the lower energy state, thereby emitting a second photon. The second photon will have the same energy and frequency as the original photon, and will also be in phase with the original photon. Thus, when the photons produced by spontaneous electron transition interact with other high energy state electrons, stimulated emission can occur so that two photons with identical characteristics are present. (Viewed as waves, the atom emits a wave having twice the amplitude as that of the original photon interacting with the atom.) If a sufficient amount of radiative recombinations are stimulated by photons, the number of photons traveling between the reflectors tends to increase, giving rise to amplification of light and lasing. The result is that coherent light builds up in the resonant cavity formed by the two mirrors, a portion of which passes through the exit mirror as the output laser beam.
Semiconductor lasers may be edge-emitting lasers or surface-emitting lasers (SELs). Edge-emitting semiconductor lasers output their radiation parallel to the wafer surface, while in SELs, the radiation output is perpendicular to the wafer surface. One type of SEL is the vertical-cavity surface-emitting laser (VCSEL). The “vertical” direction in a VCSEL is the direction perpendicular to the plane of the substrate on which the constituent layers are deposited or epitaxially grown, with “up” being typically defined as the direction of epitaxial growth. In some designs, the output laser beam is emitted out of the top side, in which case the top mirror is the exit mirror. In other designs, the laser beam is emitted from the bottom side, in which case the bottom mirror is the exit mirror. The exit mirror typically has slightly lower reflectivity than the other mirror.
VCSELs have many attractive features compared to edge-emitting lasers, such as low threshold current, single longitudinal mode, a circular output beam profile, and scalability to monolithic laser arrays. The shorter cavity resonator of the VCSEL provides for better longitudinal mode selectivity, and hence narrower linewidths. Additionally, because the output is perpendicular to the wafer surface, it is possible to test fabricated VCSELs on the wafer before extensive packaging is done, in contrast to edge-emitting lasers, which must be cut from the wafer to test the laser. Also, because the cavity resonator of the VCSEL is perpendicular to the layers, there is no need for the cleaving operation common to edge-emitting lasers.
The VCSEL structure usually consists of an active (optical gain) region sandwiched between two mirrors, such as distributed Bragg reflector (DBR) mirrors. Both EP and OP VCSEL designs are possible. The two mirrors may be referred to as a top DBR and a bottom DBR. Because the optical gain is low in a vertical cavity design, the reflectors require a high reflectivity in order to achieve a sufficient level of feedback for the device to lase. DBRs are typically formed of multiple pairs of layers referred to as mirror pairs; DBRs are sometimes referred to as mirror stacks. The DBR mirrors of a typical VCSEL can be constructed from dielectric (insulating) or semiconductor layers (or a combination of both, including metal mirror sections). The pairs of layers are formed of a material system generally consisting of two materials having different indices of refraction; for semiconductor DBRs, the layers are typically selected so that they are easily lattice matched to the other portions of the VCSEL, to permit epitaxial fabrication techniques.
For semiconductor DBRs, the number of mirror pairs per stack may range from 20-40 pairs to achieve a high percentage of reflectivity, depending on the difference between the refractive indices of the layers. A larger number of mirror pairs increases the percentage of reflected light (reflectivity). The difference between the refractive indices of the layers of the mirror pairs can be higher in dielectric DBRs, generally imparting higher reflectivity to dielectric DBRs than to semiconductor DBRs for the same number of mirror pairs and overall thickness. Conversely, in a dielectric DBR, a smaller number of mirror pairs can achieve the same reflectivity as a larger number in a semiconductor DBR. However, it is sometimes necessary or desirable to use semiconductor DBRs, despite their lower reflectivity/greater thickness, to conduct current, for example (e.g., in an EP VCSEL). Semiconductor DBRs also have higher thermal (heat) conductivity than do dielectric DBRs, making them more desirable for heat-removal purposes, other things being equal. Semiconductor DBRs may also be preferred for manufacturing reasons (e.g., a thicker DBR may be needed for support) or fabrication reasons (e.g., an epitaxial, i.e. semiconductor, DBR may be needed if other epitaxial layers need to be grown on top of the DBR).
When properly designed, these mirror pairs will cause a desired reflectivity at the laser wavelength. Typically in a VCSEL, the mirrors are designed so that the bottom DBR mirror (i.e. the one interposed between the substrate material and the active region) has nearly 100% reflectivity, while the top (exit) DBR mirror has a reflectivity that may be 98%-99.5% (depending on the details of the laser design). The partially reflective top (exit) mirror passes a portion of the coherent light built up in the resonating cavity formed by the active region and top and bottom mirrors. Of course, as noted above, in other designs, the bottom mirror may serve as the exit mirror and the top mirror has the higher reflectivity.
The transverse extent of the optical cavity must be defined in a VCSEL. This is often done by implementing a means of confinement for photons and/or electrons. Electrical or current confinement is also desirable for EP VCSELs, in which electrical current is used to provide the means of pumping the active region to achieve gain. In an EP VCSEL, for example, top and bottom electrical contacts are typically provided above and below the active region so that a pumping current can be applied through the active region. Current-confinement approaches attempt to create a current-confinement structure to confine the pumping current into a relatively small area of the active region. Confining the current in this way is also sometimes referred to as current guiding or funneling. Such a structure may be employed in a VCSEL to block current flow on an annular perimeter region of the VCSEL structure, and to guide or confine the current to a more or less cylindrical center region, which may be referred to as a current aperture or current-confinement aperture. Current confinement can provide lower threshold and higher efficiency, and can help the VCSEL operate with a single spatial mode.
Various techniques have been used to achieve electrical or current confinement, including ion implantation (irradiation) and oxidation approaches, as described in Vertical-Cavity Surface-Emitting Lasers: Design, Fabrication, Characterization, and Applications, eds. Carl W. Wilmsen, Henryk Temkin & Larry A. Coldren, Cambridge: Cambridge University Press, chapter 5 (1999). However, the active region can be as small as several microns (μm) in diameter, and the current-confinement structure aperture must be even smaller. Forming a suitable current-confinement structure can therefore be difficult to achieve, depending on the application, VCSEL size, and so forth.
In one current-confinement approach, such as that described in Jewell et al., U.S. Pat. No. 4,949,350, a post is etched through the structure. To be able to contact this structure electrically, the post should be at least tens of microns in diameter. However, for some applications, narrower current confinement may be required than the tens of microns possible with this approach.
In another technique for current confinement, patterned ion implantation into the p-doped semiconductor top DBR mirror is used to change to properties of some of the semiconductor layers to make them more resistive. This forms an apertured, relatively high-resistivity ion-implanted current-confinement region, which guides the current predominantly through an aperture. In such an approach, implantation of ions into the top DBR mirror can render the material around the laser cavity (or an aperture) relatively nonconductive, thus concentrating the injected current into the active medium. Damage, primarily crystal vacancies created by the implanted ions, compensate the free carriers leading to regions of higher resistivity. The ion dose is typically chosen to sufficiently compensate the dopant impurities in the DBR. The ion implantation energy required to achieve current confinement within a VCSEL depends upon the mass of the ion used and the implant depth desired. Thus the maximum vacancy concentration can be tailored to a specific depth within the DBR mirror.
Various ion species have been employed (e.g., H+, O+, N+, F+), although proton implants are the most common. The peak implant damage is usually designed to occur far enough above the quantum wells of the active region to avoid excessive damage to the active region. An implantation mask of photoresist or plated metal is typically employed to selectively block the ions and thus define the current-confinement structure (and therefore the laser cavity). Ion implantation and related matters are described in Y. H. Lee et al., Electr. Lett., vol. 26, no. 11, pp. 710-711 (1990); T. E. Sale, Vertical-Cavity Surface-Emitting Lasers, Research Press Ltd., pp. 117-127 (1995); and Vertical-Cavity Surface-Emitting Lasers: Design, Fabrication, Characterization, and Applications, eds. Carl W. Wilmsen, Henryk Temkin & Larry A. Coldren, Cambridge: Cambridge University Press, chapter 5 (1999).
For example, in this ion-implanted approach, light ions such as protons are implanted to relatively deep depths (e.g., about 3 μm), into selected areas of the semiconductor laser surface, damaging the semiconductor crystal lattice and thus rendering those areas relatively nonconductive (more resistive). This forms a layer having resistive sections and a less-resistive aperture portion. As another example, a 300 keV proton implant with a dose of 4×1014 cm−3 could be employed for an 850 nm VCSEL with a 20-period Al0.16Ga0.84As/AlAs DBR (2.6 μm thick). Such deep implants near the active region thus define the transverse extent of the laser cavity.
However, ion implantation generally damages the crystalline lattice structure. Sufficient ion implantation energy is therefore employed so that the ion implantation occurs deep within the structure, so that the damage is located primarily under the surface to leave a less damaged semiconductor layer at the surface for contact and current spreading. One drawback of this ion implantation technique is that there is substantial damage to the semiconductor as well as significant vertical and lateral straggle of the deeply implanted ions, which can cause reliability and performance problems with the active region. The ion straggle associated with deep implantation also limits the precision of the dimensions and location of the current aperture. For example, because of the straggle, the diameter of the current-confined region cannot be made as small as may be desired, e.g. it may not be possible to make it smaller than 10 μm in diameter. Both of these factors inhibit scaling the devices to smaller sizes. Another problem is that, as noted above, the peak implant damage is usually designed to occur far enough above the quantum wells of the active region to avoid excessive damage to the active region. Because deep implants are used, employing high implant energy which leads to significant vertical straggle, this means the implanted region has to be comparatively far away from the active region to avoid damaging it. For example, it is estimated that the maximum implant dosages must be located at least about 0.5 μm above the quantum wells, allowing significant current spreading outside the desired laser cavity. This leads to a larger threshold currents and reduced efficiency, and also places limits on how small the pumped region can be.
Another current-confinement technique is described in Chirovsky et al., U.S. Pat. No. 6,169,756 B1, the entirety of which is incorporated herein by reference. This patent describes a technique in which a lower conductivity layer (layer 18.2, in the sole FIGURE; col. 3, lines 30-50) is grown on top of the active region, and then relatively high conductivity, contact-facilitating layer (18.1) is grown on top of the lower conductivity layer. Then, a high resistivity, ion-implanted region or zone (18.3) is formed in the lower conductivity layer (18.2) by implanting this region with relatively heavy ions. However, the ion-implanted region lies within the lower conductivity layer, below another entire layer (the contact-facilitating layer). Thus, the ions must have enough energy to travel through the first, contact-facilitating layer, into the lower conductivity layer, where they implant into the ion-implanted region within the lower conductivity layer. As with the ion-implantation technique described above, this can also limit the precision of the dimensions and location of the current aperture.
One solution to the above-mentioned problems in forming a current-confining structure is to grow different parts of the VCSEL on two wafers, implant into the wafer that does not contain the active region (so as to avoid damaging the active region with ion implantation), and then bond the two wafers together. For example, such an approach is described in Y. Qian et al., Appl. Phys. Lett., vol. 71, no. 1, pp. 25-27 (1997). However, this approach also has significant manufacturing and reliability problems.
Because of problems associated with ion implantation, various oxide confinement based approaches have been explored. In some approaches, insulating oxide layers can be located immediately adjacent to the active region, reducing current spreading and thus giving rise to lower threshold currents that are typically required with conventional ion implanted VCSELs. One current-confinement approach (typically not utilized with VCSELs, however), employs an apertured, high resistivity oxide layer. It is formed by growing an oxide surface layer, then, using mask technology, etching away selected portions of the oxide. The oxide material is insulative (resistive) so current flows only into those areas of the underlying laser structure where the oxide layer has been removed (the aperture). In another approach, the native oxidation of AlGaAs layers may be used to form a resistive layer between the reflector and the active region. See for example D. Huffaker et al., Appl Phys. Lett., vol. 65, no. 1, pp. 97-99 (1994) and K. D. Choquette et al., Electr. Lett., vol. 30, no. 24, pp. 2043-2044 (1994). This approach has several drawbacks, including difficulty in manufacturing, and with control, reproducibility, and uniformity of oxidation; interference with the optical mode due to a large refractive index contrast; and a lack of a good material to oxidize for VCSELs based on materials other than GaAs (such as InP-based VCSELs for telecommunication applications). Selectively oxidized VCSELs can also have reliability problems, e.g. due to sensitivity to thermal shock/cycling.
Selective etching and undercutting of specific layers has also been proposed to form the current guiding aperture. See, for example, Scott et al., U.S. Pat. No. 5,594,751 and H. Deng et al., Electr. Lett., vol. 32, no. 10, pp. 900-901 (1996). However, this approach has significant manufacturability problems.
There is, therefore, a need for an improved current-confinement techniques and structures for guiding the current in semiconductor-based devices, such as VCSELs.